Conventionally, a technology of directly injecting fuel into a cylinder has been practically used as a technology of fuel injection to an internal-combustion-engine.
Also, recently, since it is requested to make exhaust gas cleaner and to improve fuel efficiency, power, and the like, fuel injection into a cylinder is divided into a plurality of times (multi stage injection) and downsizing to combine a supercharger with an internal-combustion-engine and to reduce displacement, is in progress. Thus, in order to deal with the minimum output to the maximum output of the internal-combustion-engine, a further expansion of a dynamic range from the minimum injection amount to the maximum injection amount of a flow characteristic of a fuel injection valve is desired.
For such an expansion of a dynamic range of a flow characteristic of a fuel injection valve, for example, it is necessary to increase the maximum injection amount by expanding a hole diameter of the fuel injection valve or to open a valve element for a very short period in a region of the minimum injection amount. Thus, it is necessary to close the valve element before it is fully opened, that is, to use an intermediate lift state.
On the other hand, in a case of opening the fuel injection valve and of injecting fuel into a cylinder, injection of high-pressure fuel and high responsivity are required. Thus, it is required to apply high voltage to the fuel injection valve and to apply high current. Thus, in a fuel injection control device to control fuel injection, a booster circuit to generate high voltage from a battery voltage is generally included.
The fuel injection control device accumulates the high voltage generated in the booster circuit into a charge accumulation element such as a capacitor and consumes the charge during the fuel injection. Then, in order to perform next fuel injection in a stable manner, the fuel injection control device completes a boosting operation with the booster circuit until the next fuel injection and recovers desired voltage. Here, when a boosted voltage becomes lower than a certain threshold, the fuel injection control device starts the boosting operation. When the voltage reaches a desired threshold, the fuel injection control device completes the boosting operation.
More specifically, as illustrated in FIG. 8, by applying current to a coil of a fuel injection valve 105′, the above-described conventional fuel injection control device 127′ controls an amount of fuel necessary for combustion. Specifically, in an internal-combustion-engine to directly inject fuel into a cylinder, in order to defeat high-pressure fuel and to deal with high responsivity, the fuel injection control device 127′ generates high voltage in an internal part thereof by performing boosting from voltage of a battery 1′ and supplies the generated high voltage to the coil of the fuel injection valve 105′ in a case of opening the fuel injection valve 105′.
More specifically, a booster circuit includes a power supply of the battery 1′, a boosting coil L1, a switching element for boosting T1, a boosting diode D1, and a boosting capacitor C1. The fuel injection control device 127′ applies current to the boosting coil L1 by turning the switching element for boosting T1 on during boosting. By turning the switching element for boosting T1 off after energy is accumulated into the boosting coil L1, the fuel injection control device 127′ accumulates the energy, which is accumulated into the boosting coil L1, into the boosting capacitor C1 through the boosting diode D1. By turning the boosting switching element T1 on/off intermittently until a predetermined voltage is reached in the boosting capacitor C1, the fuel injection control device 127′ controls the generated voltage.
The voltage of the boosting capacitor C1 is monitored, by a comparator for recognizing a stop of boosting Comp1. The fuel injection control device 127′ compares the voltage in the boosting capacitor C1 and a threshold voltage for stopping boosting Vstop indicated by 5′. When the boosted voltage reaches the threshold voltage for stopping boosting Vstop, a boosting stopping signal 3′ is output to a boosting switching control block 2′ and the boosting switching control block 2′ stops the boosting operation.
When the boosted voltage in the boosting capacitor C1 is consumed during opening of the fuel injection valve 105′, the fuel injection control device 127′ compares, with a comparator for recognizing a start of boosting Comp2, the voltage in the boosting capacitor C1 and a threshold voltage for starting boosting Vstart indicated by 6′. When the boosted voltage is equal to or lower than the threshold voltage for starting boosting Vstart, a boosting starting signal 4′ is output to the boosting switching control block 2′ and the boosting switching control block 2′ starts the boosting operation.
Moreover, the boosting switching control block 2′ monitors current, which flows in the boosting coil L1, with a current detecting resistor for monitoring a boosted current R1 and turns the switching element for boosting T1 on/off at a predetermined current threshold.
When opening the fuel injection valve 105′ by applying current thereto, the fuel injection control device 127′ monitors, with a fuel injection control block 8′, an intake air amount, the number of engine revolutions, a water temperature, and an air-fuel ratio A/F which indicate a state of an engine. Then, the fuel injection control device 127′ calculates an amount of fuel to be injected by the fuel injection valve 105′ and timing of the injection and outputs a fuel injection driving pulse illustrated in FIG. 9 to a fuel injection valve driving circuit control block 7′. Based on a profile of a current to be applied to the fuel injection valve 105′, the fuel injection valve driving circuit control block 7′ that receives the fuel injection driving pulse controls the current applied to the fuel injection valve 105′. For example, first, a valve-opening current (hereinafter, referred to as Ipeak) to defeat high-pressure fuel is applied to the fuel injection valve 105′. Then, a first holding current (hereinafter, referred to as Ihold1) is continuously applied to the fuel injection valve 105′ for a predetermined period and a second holding current (hereinafter, referred to as Ihold2) is subsequently applied thereto.
In a case of applying Ipeak to the fuel injection valve 105′, the fuel injection control device 127′ turns on switching elements T13 and T11. Accordingly, to both ends of the fuel injection valve 105′, high voltage generated in the booster circuit is supplied from the boosting capacitor C1. Here, the fuel injection valve driving circuit control block 7′ is monitored by a current detecting resistor for monitoring a fuel injection valve current R2. The boosting capacitor C1 keeps supplying the high voltage until a current value of the fuel injection valve 105′ reaches Ipeak.
Also, in a section in which Ihold1 and Ihold2 are applied to the fuel injection valve 105′, the fuel injection control device 127′ performs control to apply a predetermined current to the fuel injection valve 105′ by intermittently turning the switching element T12 on/off in a state in which the switching element T13 is turned on.
Moreover, when the voltage at both ends of the boosting capacitor C1 is decreased and becomes equal to or lower than the threshold voltage for starting boosting Vstart after the application of Ipeak, the fuel injection control device 127′ starts a boosting operation performed by the booster circuit. When the voltage reaches the threshold voltage for stopping boosting Vstop, the fuel injection control device 127′ stops the boosting operation performed by the booster circuit, keeps the boosted voltage constant, and prepares for next fuel injection.
However, in a case of applying current to the fuel injection valve 105′ for a short period (that is, for example, in case of opening valve element for very short period in region of minimum injection amount and performing fuel injection) in the fuel injection control device including the above-described conventional booster circuit, a width of a fuel injection driving pulse to drive the fuel injection valve 105′ becomes small and a decrease in the boosted voltage becomes small. Thus, as illustrated in FIG. 10, the boosted voltage does not become equal to or lower than the threshold voltage for starting boosting Vstart and current is applied, for next fuel injection, to the fuel injection valve 105′ in a state in which a condition for starting boosting is not satisfied, whereby a behavior of the fuel injection valve 105′ varies. More specifically, in the first application of current illustrated in FIG. 10, the boosted voltage reaches the threshold voltage for stopping boosting. However, since the boosted voltage is lower than the threshold voltage for stopping boosting although the boosted voltage is equal to or higher than the threshold voltage for starting boosting in the second application of current, a rising speed of the current is decreased. As a result, a problem that a difference ΔIpeak in a point reached by the current flowing in the fuel injection valve 105′ (reached current value) is generated and that a fuel injection amount varies may be generated.
For example, with respect to such a problem, a technology for driving a fuel injection valve with a prescribed voltage in a case where next fuel injection is performed before timing of starting boosting in a booster circuit is disclosed in each of PTL 1 to PTL 3.
A booster circuit for driving an injector for a vehicle which circuit is disclosed in PTL 1 includes a plurality of capacitors to accumulate a boosted voltage, uses one capacitor for each time of fuel injection, and prepares for next injection by charging a different capacitor.
Also, an internal-combustion-engine fuel injection control device disclosed in PTL 2 includes a booster circuit to boost a voltage of a power supply, a capacitor which is charged by application of the boosted voltage, an injection starting timing setting unit to set injection starting timing of fuel injected from the fuel injection valve, an injection valve driving unit to open the fuel injection valve by supplying the power charged in the capacitor to the fuel injection valve at the set injection starting timing, and a boosting control unit which controls the boosting operation performed by the booster circuit in such a manner that the voltage of the capacitor is controlled to be a predetermined target value after the fuel injection valve is opened and which raises, immediately before the injection starting timing, the controlled voltage from the target value up to a predetermined upper limit value.
Also, an internal-combustion-engine fuel injection device disclosed in PTL 3 includes a booster circuit that supplies high voltage to open a fuel injection valve that directly supplies fuel into a combustion chamber of the internal-combustion-engine, and a boosting operation control circuit that performs on/off control of a boosting operation performed by the booster circuit. Based on a signal of driving the fuel injection valve, the boosting operation control circuit starts the boosting operation in the booster circuit when application of current to the fuel injection valve is started.